The present invention relates to a vehicle transmission which includes a hill holder device and, in particular, to a hill holder device which includes a unidirectional or one-way clutch.
Various types of hill holder devices have been proposed to prevent a vehicle from rolling backward down an incline after the vehicle has come to a stop and the driver releases the brake pedal. Such hill holder devices are especially desirable on vehicles equipped with a manual transmission which is shifted by means of a foot operated clutch pedal. When a vehicle having a manual transmission is stopped on an incline, the driver will normally position one foot on the clutch pedal to disengage the transmission and the other foot on the brake pedal to hold vehicle stationary on the incline. When it is time to continue up the incline, the foot on the brake pedal must be moved to the accelerator to increase the engine speed at the same time the clutch pedal is returned to its rest position. This procedure is awkward at times because the vehicle immediately moves backward once the foot on the brake pedal is separated therefrom to release the brake system. It is in these instances wherein a hill holder device is especially desirable, since it prevents roll back of the vehicle after the brake pedal has been released.
One type of hill holder device, such as the device disclosed in U.S. Pat. No. 4,582,184, is activated when the clutch pedal is depressed as the vehicle comes to a stop on an incline with the vehicle brakes applied. The hill holder device is operative to maintain the brake pressure to the vehicle brakes after the brake pedal has been released. Brake pressure is maintained until the operator begins to release the clutch pedal.
Another type of hill holder device is disclosed in U.S. Pat. No. 4,650,046. The hill holder of this patent includes a unidirectional clutch coupled to a vehicle driveshaft. The unidirectional clutch is a sprag clutch having an inner race secured for rotation with the driveshaft, and an outer race selectively coupled to the vehicle transmission case by means of a multi-plate clutch unit. When the multi-plate clutch is fully engaged, the sprag clutch permits rotation of the vehicle driveshaft in only one direction. The multi-plate clutch is provided to control the rate of engagement of the hill holder device so as to prevent undue shocks or stress occurring in the transmission in the event the hill holder is engaged after the vehicle has begun to move backwards.